Bryan:

THAT IS WEIRD!!!!

I would trust what the IsoBar says with nothing plugged into it. What might be happening is that it might be "finding" an earth ground through some other piece of equipment that is also plugged in to the strip. You can purchase small plug-in devices at the hardware store that have three lights that tell the condition of the earth ground, and wiring of the socket. I would suggest you get one of these (they're cheap, and can amuse your friends) and verify your wiring. If you do not have a good earth ground at your outlet, the Isobar will give you little filtering benefit, since there's no ground to shunt the filtered-out crud to.

Your outlets may not have an earth ground: my 1926 house's original outlets don't. My HT room has all new electrical wiring, thank goodness.